Nearly 20 million people in China may be exposed to drinking water contaminated with arsenic, suggests a new simulation that uses certain environmental factors to calculate the risk of exposure in a particular area.

Arsenic can naturally occur in water pumped up from underground. Globally, about 140 million people drink groundwater with unsafe levels of the element, which can cause cancer and other health problems. Testing individual wells for the poison is time consuming, so a team led by...